1. Technical Field
The present invention relates to a discharge lamp lighting device, a discharge lamp lighting method, and a projector.
2. Related Art
Traditionally, as a light source for projector, for example, a discharge lamp such as an ultrahigh pressure mercury lamp, metal halide lamp or xenon lamp is used.
By the way, in a discharge lamp lighting device that controls lighting of such a discharge lamp, overshoot and undershoot may occur in the waveform of a drive current that is applied when the discharge lamp is lit.
Overshoot and undershoot are a phenomenon in which a higher current than expected flows temporarily between the electrodes of the discharge lamp, due to causes such as the circuit configuration of the discharge lamp lighting device, and in which every time the polarity of the drive current supplied to the discharge lamp is switched, the waveform of the drive current exceeds a base line that provides a stationary value, at rising and falling edges of the waveform (rectangular wave) of the drive current.
The occurrence of overshoot and undershoot causes change in illuminance of the discharge lamp and damage to the electrodes of the discharge lamp or the like. Specifically, overheating of the electrodes of the discharge lamp due to the occurrence of overshoot and undershoot causes evaporation of tungsten forming the electrodes and may cause blacking of the discharge lamp. Also, if vibration due to the occurrence of overshoot and undershoot is transmitted to the electrodes of the discharge lamp, it may cause breakdown of electrode coils.
Such damage due to the occurrence of overshoot and undershoot happens a greater number of times as the drive frequency of the discharge lamp becomes higher, and the damage is greater as the drive power (current, voltage) supplied to the discharge lamp is higher. Therefore, in the discharge lamp lighting device, the occurrence of overshoot and undershoot needs to be restrained as much as possible.
As a method for restraining the occurrence of overshoot (undershoot), for example, a method in which the current value is corrected and controlled with plural timers so as to reduce overshoot is proposed (see JP-A-9-232091). Also, a method in which an overshoot deterrent circuit is provided and made to operate every time the voltage, current, power and amount of light of the discharge lamp reach a predetermined value is proposed (see JP-A-2004-39397).
However, each of such traditional overshoot restraining methods lowers the responsiveness of the control unit to the drive waveform. If the occurrence of overshoot is restrained excessively, the waveform of the drive current and the drive power or the like cannot be controlled accurately.
In such a case, a projected image from a projector may flicker because of change in illuminance of the discharge lamp. Particularly, this phenomenon depends on the drive frequency of the discharge lamp and becomes more difficult to control as the frequency becomes higher.